The present invention relates to coatings for surface-porous substrates and more particularly to an improved coating which possesses improved surface appearance and freedom from surface imperfections.
Surface porous substrates for present purposes comprehends substrates which have surfaces replete in microfissures, pinholes, or like imperfections, typically of microdimension imperceptible by the naked eye; though larger, visually perceptible voids and imperfections may be present also. Such surface imperfections typically arise due to techniques used in manufacturing the substrate and/or due to inherent physical or chemical limitations of ingredients used in manufacturing the substrate. Prime examples of surface-porous substrates are conventional sheet molding compounds (SMC), bulk molding compounds (BMC), and like materials. These compositions, often polyester-based with additives to suppress shrinkage, can contain fiberglass or other fibrous reinforcement and upon curing of the ingredients provide an ultimate part whose surface is replete in microvoids or microfissures. It must be recognized that additional plastic materials can provide surface porosity and it is conceivable that certain types of processed fiberboard or the like additionally will be classified as surface-porous according to the precepts of the present invention. It is conceivable that certain metals even may be surface-porous too.
Manufactured parts, eg. SMC, which contain surface porosity or surface-porous substrates for present purposes, can be very small articles of manufacture such as interior molded parts of automobiles on up to entire hoods or other large parts for trucks and cars. A problem pervading manufacturers of such parts today arises in coating such parts. Typically, a primer is applied to the surface-porous part and the applied film heat cured. Unfortunately, viscosity reduction of the applied film during the initial stages of heat curing or baking can cause penetration of the coating into the surface imperfections. As the temperature of the film increases during the bake cycle, the solvent portion of the coating in the minute pores or fissures pops or blows out due to the elevated temperature of the bake cycle or air trapped in such micropores escapes, both occurrences resulting in craters or pinholes in the coating. It is not unusual for such blistered coatings to be of such poor quality that the part need be sanded and recoated a repeated number of times. Clearly, great expense and great inefficiencies arise thereby. Thus, there is a need in the art for overcoming these and other problems associated with the coating of surface-porous substrates.